1. Field of technology
The present invention relates to a thermal printer and to a control method for the thermal printer.
2. Description of Related Art
Thermal printers hold the thermal paper between the thermal print head and a platen roller and advance the paper by rotating the platen roller. The thermal print head has heating elements (dots) arrayed in a line (one dot line) across the width of the paper, and applies current to selected or all of the heating elements in this dot line to produce heat and cause the thermal paper to change color. The thermal printer prints “dots” by energizing the thermal print head while advancing the thermal paper. Torque for rotating the platen roller is transferred from a rotational drive source such as a stepping motor through a transfer mechanism (a gear train) to the platen roller.
The printing speed of a thermal printer is determined by various parameters, including the energizing voltage applied to the thermal print head, the print duty (the ratio of printed dots to the number of total dots in one dot line), the temperature of the print head, printing pattern, print data communication speed, and the amount of time required for internal data processing. These parameters are hereinafter referred to individually or collectively as “print speed control factors”. A change in one or more of the print speed control factors changes the print head energizing time and print speed. The print head energizing time and print speed are adjusted according to change in these print speed control factors in order to achieve the best print quality. The print speed of a thermal printer is equal to the paper feed rate because printing occurs while the paper is advanced.
Various control methods have been proposed for assuring good print quality when the print speed is changed based on changes in the print speed determination factors.
The control method taught in Japanese Unexamined Patent Appl. Pub. H03-231869 supplies more electrical energy to the thermal print head when the print speed is increasing or decreasing than when the print speed is constant.
The control method taught in Japanese Unexamined Patent Appl. Pub. H10-193664 measures the print head temperature and determines the print speed to control the pulse width (the thermal print head energize time and electrical energy) of a strobe signal comprising the thermal print head temperature and print speed.
The print quality of the dots printed on the thermal paper is affected by the accumulation of heat in each heating element in the print head preceding the printing of current dots. It has been discovered that by controlling the setting of the hysteresis coefficient of the thermal print head according to print speed and by changing the print speed based on the energizing history of each printed dot superior print quality can be achieved. The hysteresis coefficient can be set at multiple values based on the history of the energy applied over a period of forming multiple dots but it is preferred to set the hysteresis coefficient based on the immediately preceding application of energy to each heating element in the print head and to change the setting during the period of print deceleration.
It has been discovered that print quality varies particularly easily when the print speed decreases. When the print duty of the content to be printed is high (such as when printing solid black or during logo printing as described below), the print speed is reduced in order to avoid overheating the thermal print head and a drop in the energizing voltage, but this can also result in the print density varying.
When printing a receipt with a thermal printer in a POS terminal, for example, the store name, purchase information including the name and price of each purchased product, and a logo for the store or sales campaign are typically printed. In this case text such as the store name and the purchase information may be printed first at the beginning of the receipt, and then followed by printing a logo for a sales campaign, for example. The print duty differs greatly during logo printing of graphic data as compared to printing text, and the print speed therefore also changes. More specifically, the print duty is low and the print speed is high when printing text, and the print duty is high and the print speed is low during logo printing. There is therefore a transition from printing text to logo printing when printing both text and a logo continuously on a receipt, and the print speed decreases (gradually) at this transition from text to logo printing. As a result, when the print duty is high, the print speed is reduced so that the energizing interval (non-energized time) increases. This may be accomplished by increasing the pulse width of the strobe signal (drive signal). As shown in FIG. 11, however, the print density is unstable while the print speed is slowing, and white lines and uneven print density appear in the transition area from the deceleration range to the low speed range where the print speed is constant. As a result, print quality cannot be assured by changing only the pulse width of the strobe signal.
The print quality is easily affected by change in heat accumulation when the print speed is changed. More specifically, the cooling time of the thermal print head is shortened because the energizing interval is short during the high print speed period preceding deceleration, and because heat accumulation from the previously energized dot affects energization of the heating element in the formation of the next dot. During deceleration, however, the thermal print head cooling time increases because the energizing interval increases, and the effect of heat accumulation from the previously energized dot on the formation of the next dot is small. Controlling printing with consideration for the effect of heat accumulation has therefore been found to be necessary while the print speed is decreasing.
The present invention is directed to a thermal printer and a thermal printer control method for enabling printing with good print quality while reducing the print speed without causing streaks and uneven print density in the printed output.